Inspecting a part in immersion by means of an ultrasound beam serves to detect defects in the part such as, for example: bubbles, inclusions, etc.
The technique is implemented in a vessel filled with an acoustic coupling liquid, such as water, having an ultrasound generator, such as an electroacoustic transducer, that is immersed in the water and that is oriented towards the part for inspection perpendicularly to a surface thereof, the transducer being spaced apart from said surface of the part by a depth of water referred to as the “water column”.
The transducer emits a beam of ultrasound waves which is partially reflected on the surface of the part and partially transmitted to the inside of the part, the transmitted ultrasound waves possibly encountering on their trajectories internal defects that are capable of reflecting them. The echoes coming from such reflections are picked up by the transducer, which also acts as a receiver, and the amplitudes of the echoes represent the magnitudes of the detected defects.
In practice, inspecting a part includes relative movement between the part and the transducer or a series of transducers so as to scan the part by means of one or more ultrasound beams.
When the ultrasound beam is moved progressively towards an edge of the part, an ever-increasing fraction of the section of the beam at the surface of the part lies beyond the edge of the part such that a fraction of the energy of the beam is emitted outside the part and will not be reflected by a defect in that portion of the part that is covered by the beam.
Consequently, a defect will be detected in the form of an echo, but of amplitude that becomes progressively weaker as the beam approaches the edge of the part, with said amplitude dropping to zero when the beam no longer encounters the part.
The portion of the part in which the echo amplitude on the beam being reflected by a defect decreases because of this phenomenon is a zone that cannot be inspected by known methods and is referred to as a “shadow zone” or as a “lateral dead zone” of the part. The known technique does not make it possible to determine the exact extent of said dead zone at arbitrary depth in the part, and it is therefore necessary as a precaution to give it a maximum value, e.g. 12 millimeters (mm) even though it might be much smaller in reality, e.g. half that, which corresponds to ignoring a relatively large amount of matter.